A through tubing bridge plug (TTBP hereinafter) is a structure which is typically placed in a producing well to isolate the lower portions of the well from upper portions of the well. In a typical case, a tubing string delivering production fluids from lower portions of the well is selectively plugged to isolate lower portions of the well. This might occur in one instance where a lower horizon is producing great quantities of water but is not otherwise commercially valuable because it is producing excessive water. Often, there will be high horizons of interest which can produce oil and gas in commercial paying quantities. In this instance, a TTBP will be positioned at a selected depth in the well. In effect, it forms a permanent plug which isolates the lower portions of the well. This then enables the well to be reconfigured to obtain production from the higher horizon.
In the foregoing exemplary problem, the TTBP is positioned at the selected depth and is anchored there. In the ordinary course, installation of a TTBP requires that the device be located at a specific depth in the well. When so located, it must hold that location or depth. Typically, it must hold for several hours, perhaps as long as 36 hours. This is necessary so that time will permit sand and cement to be added above the TTBP to form the plug. Moreover, it must cure, the curing requiring several hours. Curing must be accomplished notwithstanding flow of well fluid from the formations below the TTBP. Fortunately, the TTBP has a bypass flow route in the way of a controllable valve connector to end a tubing through the TTBP. As an example, the TTBP is installed at the requisite horizon whereupon a multileaf basket is deployed, opening upwardly, to receive sand and cement in the basket. The tubing extends up through and past the basket. The tubing has openings below the basket to receive well fluid from below which fluid is conducted upwardly through the tubing and past the basket. This provides pressure relief so that pressure does not build up below the TTBP until after the cement has cured. The tubing thus is selectively opened and closed by a valve installed in the tubing.
The closure of the valve and hence blocking flow from horizons below the TTBP creates a pressure differential acting across the area of the tubing, thereby creating a force which may move the TTBP. The present apparatus sets forth a TTBP anchor system which anchors so that the plug will not be moved, nor the TTBP dislodged. The present apparatus thus provides a positive mechanical anchor which expands within the anchor, taking a bite in the tubing and having a toggle action which assures that the anchor will hold both at the time of installation before the plug cures and long thereafter. In the present apparatus, the anchor is located below the basket in the preferred embodiment to centralize and anchor the basket thereabove. This fixes in a concentric location the tubing extending upwardly through the TTBP basket. This apparatus is deployed by forcing a toggle link arrangement radially outwardly against the wall of the tubing. This assures that the device can then be left because the toggle links will deflect and deploy serrated teeth which bite into the tubing wall. The toggle links are held into this position by means of a spring bearing on the toggle mechanism, the spring holding even after the TTBP has been left in place. A method of operation utilizing the toggle mechanism is also disclosed.
In general terms, there is very little gripping accomplished by a cured plug formed above a TTBP. It is possible for the entire plug to slip. As described above it is possible to set a toggle type anchor below the TTBP which accomplishes a strong grip with the surrounding casing. In addition to that, the anchor of the present disclosure can be replicated at multiple locations above the TTBP. Perhaps some discussion would assist in this. As one example, consider a plug which is exposed to a significant differential pressure which might otherwise cause the cured plug to move. The toggle type mechanism described above is set below the TTBP. In addition to that, one or more of the toggle mechanisms can be placed at spaced locations above the TTBP and the first plug which is formed. By placing additional quantities of materials above the first cast, several toggle mechanisms can be submerged in the completed plug. The completed plug after several pours might be quite long and can encase one or more toggle mechanisms which lock the completed plug in location. The toggle mechanism thus functions as a separate or isolated anchor which is cast in place to secure the plug against high pressure differentials which might otherwise force the plug to break free of the grip.
An important advantage arises from this. One weakness of such a toggle mechanism arises in the long linkages which are required to achieve the set position. These long linkages are subject to buckling. That is, the failure mode is buckling rather than failure in compression or tension of the structural members. Once the plug is cast around them, buckling is prevented by the cast member. Accordingly, the toggle mechanism will endure much longer and hold against greater forces in the well.